Problem: What is the least common multiple of 9 and 21? Another way to say this is: $\lcm(9, 21) = {?}$
Answer: The least common multiple is the smallest number that is a multiple of 9 and 21. We know that 9 x 21 (or 189) is a common multiple, but is it the least common multiple? Write out the multiples of 21 until we find a number divisible by 9. 21, 42, 63, So, 63 is the least common multiple of 9 and 21.